In mammals the corpus luteum (CL) of the ovary secretes progesterone, the hormone that maintains the uterus in a state adequate for pregnancy. The requirement for a fully functional CL may continue throughout pregnancy, as in the pig and cow, or end before term, as in the ewe, where the placenta and/or adrenal glands may assume the production of progesterone after 50 days of pregnancy (BAZER and FIRST, 1983). The demise of the corpus luteum, through a process called luteolysis, is mediated by uterine secretion of prostaglandin F.sub.2 .alpha. (PGF.sub.2 .alpha.). It ensures termination of pregnancy.
The conceptus within the uterus is believed to produce a substance or substances which directly or indirectly prolong the lifespan of the corpus luteum and prevent a return to ovarian cyclicity. This phenomenon is known as `maternal recognition of pregnancy` (SHORT, 1969). These substances are proteins referred to as trophoblast antiluteolysin, trophoblastin or trophoblast protein-1 (ROWSON and MOOR 1967, MARTAL et al 1979, BAZER et al 1986).
Ovine trophoblast protein-1 (oTP-1) is the major secretory product synthesized by the sheep conceptus between day 13 and 21 of pregnancy (GODKIN et al 1982). The protein consists of 3-4 isoelectric variants (with isoelectric points between 5.5 and 5.8) with a relative molecular weight of 17,000 to 21,000 (HANSEN et al 1985). Infusion of oTP-1 into the uterine lumen of non-pregnant ewes prevents CL regression and extends the estrous cycle (GODKIN et al 1984). The mechanism of action is not fully understood but oTP-1 has been shown to act on the uterine endometrium, altering protein secretion and phospholipid metabolism (GODKIN, BAZER and ROBERTS 1984). Such an effect would tend to reduce prostaglandin F.sub.2 .alpha. synthesis.
The cow conceptus secretes related molecules that cross react with antiserum to oTP-1. They are secreted by the cow conceptus between day 15-25 of pregnancy. The family of molecules is called bovine trophoblast protein-1 (bTP-1) and includes two structurally related glycoproteins of 22,000 and 24,000 molecular weight, each of which is present in three isoforms that arise from different messages (HELMER et al. 1987; ANTHONY et al., 1988; HELMER et al., 1988).
Recently a cDNA for oTP-1 has been sequenced (IMAKAWA et al 1987). Its primary amino-acid sequence inferred from the nucleotide sequence has 45-55% homology with a range of human, bovine, mouse, rat and pig interferons of the alpha family. Similarly, cDNA clones of the bovine trophoblast proteins (bTP-1) have been obtained and sequenced (Roberts and Imakawa, unpublished information). The sequences of mature oTP-1 and bTP-1 are each 172 amino acids in length. bTP-1 is 79.7% identical with oTP-1 within the region of the mature protein. oTP-1 is 70.3% homologous in sequence with rbINF.alpha..sub.II, whereas bTP-1 is only 69.2% homologous. The signal sequences for bTP-1 and oTP-1 differ in one residue out of 23. The signal sequences of bTP-1 and rBoIFN.alpha..sub.II are identical. Preliminary observations suggest that different isolates of oTP-1 and bTP-1 are closely related and differ in sequence by few amino acids (Roberts and Imakawa, unpublished observations). It appears therefore that trophoblast proteins represent a group of proteins with high sequence conversation. This is in contrast to the interferons alpha derived from leukocytes which have been shown to differ widely in structure in all species tested.
Interferons have generally been named after the species of animal producing them (for example, human, ovine, bovine, murine, etc.), the type of cell involved in their production (for example, leukocyte, lymphoblastoid, fibroblast) and, occasionally, the type of the agent inducing their production (for example, virus, immune). A further classification refers to Type I or Type II comprehending virus and nucleic acid induced interferons. A recent classification refers to alpha, beta and gamma which correspond to previous designations of leukocyte, fibroblast, and type II (immune) interferons, respectively. The interferon employed herein is identified simply by animal species and the cell type producing it (for example, bovine interferon alpha).
The preparation of mammalian interferons of the alpha family is described in the art. The isolation of mammalian IFN.alpha. and/or the preparation thereof by means of recombinant DNA technology is described, for example, in the following patents and patent applications: U.S. Pat. Nos. 4,273,703; 4,328,207; EP-002,375; U.S. Pat. Nos. 4,262,090; 4,530,901; 4,414,150; EP-0,088,622; JP-58/224,690; U.S. Pat. Nos. 4,582,800 and 3,951,740. Interferons useful in accordance with the present invention are not limited to those alpha interferons described in the above cited literature. Further useful alpha interferons are known in the art.
Throughout the present specification an interferon of the alpha family (IFN .alpha.) is understood to represent a leukocyte-derived interferon or an interferon produced by means of recombinant DNA technology which still has the structural, immunological and antiviral and other biological properties of a leukocyte-derived interferon.
In contrast thereto, the trophoblast proteins (antiluteolytic proteins), e.g. oTP-1 and bTP-1, are conceptus--derived proteins. Despite certain homologies referred to above the relationship between trophoblast proteins and interferons is not yet fully clarified. It has been shown that with respect to in-vitro actions on endometrial protein and prostaglandin metabolism, actions of bTP-1 differ markedly from those of IFN-.alpha. (Thatcher et al., 1988).
It is an objective of the present invention to provide a method for enhancing fertility in female mammals and to prolong the lifespan of mammalian corpus luteum.
These and other objectives will be apparent from the following description of the invention.